Measuring change for longitudinal study

Hi all

Is there an analysis method to measure the significance of magnitude change overtime (longitudinal study)?

For example: Person A weighted 30kg in Year 1, 40kg in Year 2, 35kg in Year 3; Person B weighted 30kg in Year 1, 35kg in Year 2, 40kg in Year 3.

How do I know whether the magnitude change over the 3 years is significant for person A/B? Or which person has more significant change over the years?

I don't even know where to start for looking such information- any advice is appreciated.